Optical disk apparatus and track access control method utilizing positioned shift between scanning beams

ABSTRACT

An optical disk apparatus includes first and second objective lenses, a carriage, a moving mechanism, a fixed optical system, a main control section, track access control circuits, a first positional shift correction circuit, a second positional shift correction circuit, first and second driving members, an arithmetic circuit, and a correction member. The first and second driving members slightly drive the first and second objective lenses in a track crossing direction in accordance with the first and second positional shift correction signals, respectively. The first and second positional shift correction circuits count first and second track position signals to output first and second positional error signals with respect to a target track, and output the first and second positional shift correction signals to the first and second driving members based on the first and second positional error signals and the first and second track position signals, respectively. The arithmetic circuit calculates a difference between the first and second positional error signals to output an inter-beam positional shift signal. The correction member corrects one of the first and second positional shift correction signals based on the inter-beam positional shift signal.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disk apparatus for opticallyrecording and reproducing information and, more particularly, to a trackaccess control method in which a plurality of beam irradiation lenses(objective lenses) are arranged in series with each other in arecording/reproducing direction, and information is recorded at a highspeed by erasing/recording information with a preceding beam and byrecording/reproducing information with a following beam, and an opticaldisk apparatus using this method.

In an optical disk apparatus for optically writing and readinginformation in and from a disk-like recording carrier, and particularlya magneto-optical disk apparatus having an information rewrite function,two beam irradiation cycles, i.e., a cycle for irradiating a light beamto erase information, and a cycle for irradiating a modulated light beamfor recording new information are required.

To eliminate a cumbersome recording procedure and shorten the processingtime in such a magneto-optical disk apparatus, an optical disk or amagneto-optical disk apparatus separately using an information erasingbeam and an information recording/reading beam has been proposed.

For example, an arrangement in which two light beams are emitted throughone objective lens to use one for erasing information and the other forrecording/reading information is disclosed in Japanese Patent Laid-OpenNo. 61-250846.

Although the arrangement in which the erasing beam and the recordingbeam are close to each other is effective for a phase change typerewritable optical disk which requires no external magnetic field forrecording and erasing, this arrangement is difficult to be applied tothe magneto-optical disk apparatus. In the magneto-optical diskapparatus in which recording and erasing magnetic fields must beinverted, positions where the recording and erasing magnetic fieldsapplied to a medium must be spaced apart from each other by a certaindistance. It is preferable to irradiate beams focused by correspondingobjective lenses on erasing and recording positions, respectively.

A magneto-optical disk apparatus having such light beams is disclosedin, e.g., Japanese Patent Laid-Open No. 3-263638. In this example, ahead for forming a spot for recording/erasing information, and a headfor forming a spot for reproducing information are mounted on the samecarriage, and the erasing/recording spot precedes the reproducing spoton the same track by a predetermined distance.

In the optical disk apparatus in which the separate optical heads orobjective lenses are arranged in order to irradiate two beams onpositions relatively apart from each other on a medium, and are mountedon the same carriage to move the beams in the radial direction of thedisk (direction perpendicular to a track), the relative positions of thetwo beams must be kept almost constant with respect to a track on thedisk. Particularly, in recording and reading operations, the two beamsmust be present on the same track.

To allow the two beams to rapidly reach the same track upon completionof a track access, the two beams desirably move following almost thesame trace without greatly shifting the relative positions of the twobeams during the inter-track movement of the beams (track access).

On the other hand, a preferable track access control method for theapparatus having the two objective lenses or optical heads mounted onthe same carriage has not been attained.

One of easily assumed methods is a method of stopping (activating a stopservo) the objective lenses with respect to the reference axis of acarriage or head in a track access to move the carriage with referenceto a track position signal detected from either beam.

In this method, however, the relative positional precision of the twobeams during the track access depends on the positional precision of thelens stop servo and the servo characteristics, and a precision enough toposition the beams on the same track cannot be expected. Therefore, atthe end of the carriage movement, the positions of the two beams aregreatly shifted from each other. To align the two beams on a targettrack, the beams must be precisely sought (track jump seek), requiring along period of time.

In the optical disk apparatus in which a plurality of objective lensesare mounted on the same carriage to simultaneously perform erasing andrecording operations, the time required for a track access in whichbeams are moved to a target track is greatly prolonged.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical diskapparatus capable of correctly moving two beams to the same target trackat a high speed while reducing a positional shift therebetween whenobjective lenses mounted on a carriage to irradiate the two beams aretransferred to another same track, and a track access method for theoptical disk apparatus.

In order to achieve the above object, according to the presentinvention, there is provided an optical disk apparatus comprising firstand second objective lenses respectively for outputting first and secondlight beams on the same track on an optical disk, the first and secondbeams separately recording, reproducing, and erasing information on thesame track, first and second driving means respectively for slightlydriving the first and second objective lenses in a track crossingdirection in accordance with a first and a second positional shiftcorrection signals, a carriage on which the first and second objectivelenses are mounted, a moving mechanism for moving the carriage betweentracks, a fixed optical system for emitting the first and second beamson the optical disk and detecting the beams reflected by the opticaldisk through the first and second objective lenses, and extracting firstand second track position signals from the detected reflected beams ofthe first and second beams, main control means for controlling anoperation of the moving mechanism, and controlling an informationrecording/reproducing/erasing operation using the first and secondobjective lenses through the fixed optical system, track access controlmeans for controlling the movement of the carriage between the tracksbased on the first and second track position signals from the fixedoptical system, first positional shift correction means for counting thefirst track position signal from the fixed optical system to output afirst positional error signal with respect to a target track, andoutputting the first positional shift correction signal to the firstdriving means based on the first positional error signal and the firsttrack position signal, second positional shift correction means forcounting the second track position signal from the fixed optical systemto output a second positional error signal with respect to the targettrack, and outputting the second positional shift correction signal tothe second driving means based on the second positional error signal andthe second track position signal, arithmetic means for calculating adifference between the first and second positional error signals fromthe first and second positional shift correction means to output aninter-beam positional shift signal, and correction means for correctingone of the first and second positional shift correction signals from thefirst and second positional shift correction means based on theinter-beam positional shift signal from the arithmetic means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing an optical disk apparatusaccording to an embodiment of the present invention;

FIG. 2 is a block diagram showing an arrangement of a track accesscontrol circuit 7A shown in FIG. 1;

FIG. 3 is a block diagram showing an arrangement of a track accesscontrol circuit 7B shown in FIG. 1;

FIG. 4 is a block diagram showing an arrangement of a first positionalshift correction circuit shown in FIG. 1;

FIG. 5 is a block diagram showing an arrangement of a second positionalshift correction circuit shown in FIG. 1; and

FIG. 6A is a chart illustrating waveforms of the speeds of the first andsecond beam spots, FIG. 6B is a chart showing waveforms of changes intrack error signals e and n of the first and second beams during a trackaccess, and FIG. 6C is a chart showing a waveform of a positional shiftsignal q indicating a positional shift between the two beams.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below withreference to FIGS. 1 to 6.

Referring to FIG. 1, reference numeral 1 denotes an optical disk; and 2,tracks on the optical disk 1. First and second objective lenses 11 and14 are disposed on the optical disk 1 such that the objective lenses 11and 14 can be reciprocally moved from the peripheral end portion of theoptical disk 1 toward its central portion to irradiate two beams on thesame track 2.

An optical disk apparatus of this embodiment comprises a carriageactuator 5 serving as a moving mechanism for moving a carriage 4, onwhich the two objective lenses 11 and 14 are mounted, to the same trackposition, a main control section 6 for controlling the operation of thecarriage actuator 5, and for controlling the write/read access ofpredetermined information by the first and second objective lenses 11and 14 through a fixed optical system 3, and track access controlcircuits 7A and 7B serving as track access control means for controllingthe movement of the carriage 4 between the tracks 2 on the basis oftrack position information obtained by the fixed optical system 3.

In this embodiment shown in FIGS. 1 to 6, an error which occurs inmoving two light beams between the tracks in the optical disk apparatuswill be minimized. The important feature of this embodiment lies in thatpositional errors of the light beams with respect to a target track aredetected, a difference between the positional errors is calculated touse the result as a positional shift signal indicating the positionalshift between the beams, and this positional shift signal is added as acorrection value in moving one of the light beams.

To realize this, the track access control circuits 7A and 7B of thisembodiment are respectively equipped with first and second positionalshift correction circuits 8 and 9 for independently, slightly adjustingthe positions of the first and second objective lenses 11 and 14, andthe like.

The first positional shift correction circuit 8 has a function ofcounting first track position signals (track pulses) b which aredetected by the first light beam, obtained by the fixed optical system3, and supplied from the track access control circuit 7A, therebyoutputting a first positional error signal e with respect to a targettrack. In addition, the first positional shift correction circuit 8 alsohas a function of outputting a positional shift correction signal r forthe first objective lens on the basis of the first positional errorsignal e and the first track position signal b.

The second positional shift correction circuit 9 has a function ofcounting second track position signals (track pulses) k which aredetected by the second light beam, obtained by the fixed optical system3, and supplied from the track access control circuit 7B, therebyoutputting a second positional error signal n with respect to the targettrack. In addition, the second positional shift correction circuit 9also has a function of outputting a positional shift correction signal sfor the second objective lens on the basis of the second positionalerror signal n and the second track position signal k.

The carriage 4 is independently equipped with lens actuators 12 and 15serving as first and second driving means which are driven andcontrolled by the corresponding first and second positional shiftcorrection circuits 8 and 9 to slightly drive the correspondingobjective lenses 11 and 14 in the track crossing direction.

Further, the optical disk apparatus is equipped with an inter-beam trackdifference arithmetic device 50 for calculating a difference between thefirst positional error signal e output from the first positional shiftcorrection circuit 8 and the second positional error signal n outputfrom the second positional shift correction circuit to form and outputan inter-beam positional shift signal q, and an addition circuit 50A forcorrecting the output s from the second positional shift correctioncircuit 9 on the basis of the output signal q from the inter-beam trackdifference arithmetic device 50.

FIG. 1 shows a track access control apparatus in the optical diskapparatus according to the embodiment of the present invention. The twoobjective lenses 11 and 14 are mounted on the carriage (movingmechanism) movable in the radial direction of the optical disk 1. Beamsemitted through the objective lenses are condensed to be irradiated onthe recording surface of the optical disk 1 as first and second beamspots 10 and 13, respectively. Note that each light beam is focused andcondensed on the recording surface of the disk by focus control (focusservo).

The disk 1 is rotated by a spindle motor (not shown) in a rotationaldirection indicated by an arrow A. A large number of information tracks2 are formed on the recording surface of the disk 1. In an informationrecord/read access, the beam spots 10 and 13 are controlled to follow atarget track 2.

The lens actuators 12 and 15 for moving the corresponding objectivelenses in the radial direction of the disk are mounted on the carriage4. The objective lenses 11 and 14 are coupled to the corresponding lensactuators 12 and 15.

A pair of laser beam sources for generating two light beams, and a pairof photodetectors for receiving the two beams reflected by the disk arearranged not on the carriage but inside the fixed optical system 3. Thefixed optical system 3 is fixed with respect to the optical disk 1, andonly light beams reciprocate between the fixed optical system 3 and thecarriage 4. A first track error detector 20 attached to the fixedoptical system 3 receives the reflected beam of the first beam spot 10to detect a positional shift of the first beam spot with respect to aninformation track.

The track error detector 20 is constituted by, e.g., a two-dividedphotodetector. The track error detector 20 detects a change in reflectedbeam (change in intensity distribution) caused by a positional shift ofa beam spot with respect to an information track to output a currentcorresponding to the positional shift of the beam with respect to thecenter of the track to the track access control circuit 7A. A tracksignal detection circuit 21 in the track access control circuit 7Areceives the current signal from the track error detector 20 to generatea track error signal a indicating the positional shift amount of thebeam spot 10 with respect to the center of the track, as shown in FIG.2.

On the other hand, a lens position detector 16 is attached to the lensactuator 12 to detect the movement of the objective lens 11 in theradial direction of the disk 1. In this embodiment, the lens positiondetector 16 is constituted by a reflection position sensor in which anLED element is arranged at the center, and photodiodes are arranged onthe two sides. The output terminal of the lens position detector 16 isconnected to a lens positional shift detection circuit 28.

The lens positional shift detection circuit 28 generates a lens positionsignal g indicating a positional shift of the objective lens 11 withrespect to the reference position of the carriage 4. The lens positionsignal g is input to a power amplifier 29 through a phase compensationcircuit 51. The power amplifier 29 amplifies this input signal. Theamplified signal is supplied to the carriage actuator 5 to drive it soas to correct the positional shift between the carriage 4 and theobjective lens 11, i.e., to reduce the lens position signal to almost"0". Upon reception of this lens position signal, a loop for driving thecarriage 4 is activated to cause the carriage 4 to follow the movementof the objective lens 11.

The carriage 4 follows the lens 11 while the beam spot 10 moves betweenthe tracks. When the beam spot 10, i.e., the objective lens 11 moves inthe radial direction of the disk 1, the carriage 4 also follows thismovement and moves in the radial direction.

The track error signal a output from the track signal detection circuit21 is input to a tracking servo circuit 22 and a track pulse generationcircuit 23, as shown in FIG. 2. The tracking servo circuit 22 is acircuit for keeping the beam spot 10 on a current track (to cause thebeam spot 10 to follow the track). The tracking servo circuit 22 has afunction of correcting the phase of the track error signal a, amplifyingit, and supplying a driving current to the lens actuator.

The track signal detection circuit 21, the tracking servo circuit 22,and the track pulse generation circuit 23 constitute the track accesscontrol circuit 7A.

Upon reception of the track error signal a, the track pulse generationcircuit 23 detects its zero-crossing point to output a track pulse bindicating that the light beam crosses a track.

A speed signal generator 24, in the first positional shift correctioncircuit 8, which receives the output from the track pulse generationcircuit 23, is constituted by, e.g., an F/V conversion(frequency/voltage conversion) circuit, as shown in FIG. 4. Uponreception of the track pulse b, the speed signal generator 24 outputs aspeed signal c indicating the speed of the beam spot 10 moving acrosstracks.

On the other hand, a track error signal generator 25 which also receivesthe output from the track pulse generation circuit 23 externallyreceives a target movement amount signal α indicating a distance fromthe current position of the beam spot to a target track (i.e., thenumber of tracks to be crossed) at the start of moving the beam spot 10,as shown in FIG. 4. The track error signal generator 25 counts down thetarget movement amount signal α in accordance with the track pulses bgenerated upon the movement of the beam spot 10 to output the trackerror signal e indicating a remaining distance (track number) to thetarget track for the moving light beam spot 10. A reference speed signalgenerator 26 generates a reference speed signal f for defining themoving speed of the beam spot 10 on the basis of the track error signale. A difference between the reference speed signal f and the speedsignal c is calculated by a first subtracter 8a to output the positionalshift correction signal r.

The speed signal generator 24, the track error signal generator 25, thereference speed signal generator 26, and the first subtracter 8aconstitute the first positional shift correction circuit 8.

The positional shift correction signal r from the first subtracter 8a isinput to a power amplifier 27. The power amplifier 27 amplifies thepositional shift correction signal r to supply it as a correction signalfor driving the lens actuator 12 in the target track direction to thelens actuator 12.

With such a control operation, the beam spot 10 is moved to the targettrack. The radial movement of the lens actuator 12 is detected by thelens position detector 16, and the carriage 4 is driven in response tothe lens position signal g so as to follow the movement of the objectivelens 11, as described above. The carriage 4 is therefore moved to thetarget track direction at the same time the beam spot 10 is moved.

On the other hand, the second beam spot 13 is required to perform almostthe same movement as that of the first beam spot 10 and to reach thesame target track.

For this purpose, the fixed optical system 3 is equipped with a secondtrack error detector 30 to detect a positional shift of the second beamspot 13 with respect to an information track.

As shown in FIG. 3, a track signal detection circuit 31 in the trackaccess control circuit 7B, which receives an output from the secondtrack error detector 30, receives a current signal from the track errordetector 30 to generate a track error signal h indicating the positionalshift amount of the beam spot 13 with respect to the center of thetrack. The track error signal h is input to a tracking servo circuit 32and a track pulse generation circuit 33. The tracking servo circuit 32is a circuit for keeping the beam spot 13 on a current track (to causethe beam spot 13 to follow the track). The tracking servo circuit 32 hasa function of correcting the phase of the track error signal h,amplifying it, and supplying a driving current to the lens actuator 15.

The track signal detection circuit 31, the tracking servo circuit 32,and the track pulse generation circuit 33 constitute the track accesscontrol circuit 7B.

The track pulse generation circuit 33 which receives the track errorsignal h from the track signal detection circuit 31 detects itszero-crossing point upon reception of the track error signal h to outputa track pulse k indicating that the beam spot 13 crosses a track.

A speed signal generator 34, in the second positional shift correctioncircuit 9, which receives the output from the track pulse generationcircuit 33, is constituted by, e.g., an F/V conversion(frequency/voltage conversion) circuit, as shown in FIG. 5. Uponreception of the track pulse k, the speed signal generator 34 outputs aspeed signal m indicating the speed of the beam spot 13 moving acrosstracks. At the start of moving the beam spots 10 and 13, a track errorsignal generator 35 which also receives the output from the track pulsegeneration circuit 33 receives the target movement amount signal αindicating a distance from the current position of the beam spot 13 tothe target track (i.e., the number of tracks to be crossed) at the sametime when the track error signal generator 25 receives the targetmovement amount signal α. The track error signal generator 35 countsdown the target movement amount signal α in accordance with the trackpulses k generated upon the movement of the beam spot 13 to output thetrack error signal n indicating a remaining distance (track number) tothe target track for the moving light beam spot 13. A reference speedsignal generator 36 generates a reference speed signal p for definingthe moving speed of the beam spot 13 on the basis of the track errorsignal n.

A difference between the reference speed signal p and the speed signal mis calculated by a second subtracter 9a to output the positional shiftcorrection signal s.

The speed signal generator 34, the track error signal generator 35, thereference speed signal generator 36, and the second subtracter 9aconstitute the second positional shift correction circuit 9.

The positional shift correction signal s from the second subtracter 9ais input to a power amplifier 37. The power amplifier 37 amplifies thepositional shift correction signal s to supply it as a correction signalfor driving the lens actuator 15 in the target track direction to thelens actuator 15.

With such a control operation, the beam spot 13 is moved to the targettrack. In this manner, the movement control method and the arrangementof the control system for the beam spot 13 are almost the same as thosefor the beam spot 10. Therefore, the beam spot 13 (i.e., the objectivelens 14) and the beam spot 10 (i.e., the objective lens 11) performalmost the same operation to move to the target track.

On the other hand, the characteristics of the lens actuator 12 areslightly different from those of the lens actuator 15. In addition, thespeed signals c and m respectively generated by the speed signalgenerators 24 and 34 have a slight error therebetween.

If, therefore, the movements of the two beam spots are separatelycontrolled by the two control systems arranged separately, the movingspeeds of the two beam spots may be slightly different from each other.As a result, a positional shift between the two beams 10 and 13increases with a lapse of moving time. If this relative positional shiftis not corrected, the position of the objective lens 14 is shifted fromthe objective lens 11. In this case, the position of the objective lens14 is greatly shifted from the carriage 4 because the carriage 4 ismoved to follow the objective lens 11.

If the position of the objective lens 14 is greatly shifted from thecarriage 4, the condensing properties of the beam spots are degraded tomake correct detection of a track error signal difficult, resulting inunstable movement control (speed control) of the beams. In addition, thepositional shift between the two beams is greatly increased at the endof the movement, so that a moving operation for correction must beperformed to reach the target track.

To reduce a positional shift between two beam spots in moving acrosstracks, a feedback loop for detecting the positional shift between thebeams, and adjusting the movements of the beams so as to reduce thepositional shift is added to the track access control system in thisembodiment.

More specifically, the track error signals e and n output from thecorresponding track error signal generators 25 and 35 respectively shownin FIGS. 4 and 5 are input to the inter-beam track difference arithmeticdevice 50. The track error signal e indicates a positional shift(distance) of the beam spot 10 with respect to the target track, whilethe track error signal n indicates a distance to the target track forthe beam spot 13. A difference between the two track error signals e andm indicates a positional shift between the two beams.

The inter-beam track difference arithmetic device 50 calculates thedifference between the track error signals e and n to output theinter-beam positional shift signal q indicating a relative positionalshift between the two beams 10 and 13. The gain of the inter-beampositional shift signal q is adjusted to a proper level by a leveladjustment circuit 38 and input to the power amplifier 37 so as to beadded to the signal s (speed error signal) indicating a differencebetween the reference speed signal p and the speed signal m.

More specifically, the power amplifier 37 receives the signalcorresponding to the positional shift of the beam spot 13 with respectto the beam 10 in addition to the signal s indicating an error of thespeed signal m with respect to the reference speed signal p (i.e., thesignal corresponding to a speed error of the beam spot 13 with respectto a target speed). Upon reception of these signals, the power amplifier37 supplies a driving signal for correcting the positional shift betweenthe beam spots 10 and 13 to the lens actuator 15. That is, the objectivelens 14 is accelerated when the beam 13 has a delay with respect to thebeam spot 10, while the objective lens 14 is slightly slowed down whenthe beam 13 precedes the beam 10.

With such a correction operation for a relative positional shift betweenbeams, the two beam spots 10 and 13 move following almost the same tracein moving across tracks while keeping almost the same positionalrelationship, and reach almost the same track.

The moving speed of the beam spot 10 is preferably equal to that of thebeam spot 13, as much as possible. The reference speed signals f and mmay be identical to each other.

More specifically, the reference speed signal generator 36 shown in FIG.5 for the second beam spot is not necessarily arranged, and thereference speed signal f for the first beam may be used as a referencespeed signal for the second beam spot. In this case, the reference speedsignal generator 36 is unnecessary. The reference speed signal f outputfrom the reference speed signal generator 26 also serves as a referencefor the speed signal m, and a difference between the reference speedsignal f and the speed signal m is input to the power amplifier 37.

In the above description, the control apparatus of the present inventionis constituted by circuit blocks having individual functions. Thisarrangement, however, is only an example for facilitating theexplanation of the function and operation of the control apparatus ofthe present invention. The main part of the functions of the apparatusaccording to the present invention may be realized by program processingwith a digital signal processor (DSP).

For example, in the arrangements shown in FIGS. 1, 4, and 5, the trackerror signal generators 25 and 35, the reference speed signal generators26 and 36, the speed signal generators 24 and 34, the inter-beam trackdifference arithmetic device 50, the arithmetic operation of calculatingan error between the reference speed signal f and the speed signal c,the arithmetic operation of calculating an error between the referencespeed signal p and the speed signal m, and the arithmetic operation oflevel-adjusting the inter-beam positional shift signal q to add theobtained signal to the difference between the reference speed signal pand the speed signal m may be realized by the digital signal processor.In this case, a D/A converter for converting a digital signal outputfrom this digital signal processor into an analog signal (voltagesignal) capable of being input to the power amplifiers 27 and 37 isarranged between the power amplifiers 27 and 37.

An operation will be described below with reference to FIGS. 6A to 6C.

FIG. 6A shows schematic waveforms of the speeds of the first and secondbeam spots 10 and 13 during a beam moving operation. FIG. 6B shows anexample of a waveform of the track error signal e (converted into ananalog value) corresponding to a positional error (distance) of thefirst beam spot 10 with respect to a target track, and a waveform of thetrack error signal n (converted into an analog value) corresponding to apositional error of the second beam spot 13 with respect to the targettrack.

In moving the beams between tracks (track access), the lens actuatorsare accelerated in the target track direction in accordance with thereference speed signals to gradually increase the speeds of the beamspots. When the speeds of the beams reach maximum speed level V_(H), theacceleration is stopped, and the beams travel at a constant speed(V_(H)). When the beam spots come near the target track with apredetermined distance, the reference speed signals are reduced to slowdown the beam spots.

When the beam spots reach the target track, the level of the track errorsignals goes to "0", and the reference speed signals are also set to be"0" to stop the movement of the beam spots. In the movement betweentracks, if the speeds and traces of the two beams completely coincidewith each other, the two beam spots 10 and 13 move without any relativepositional shift. In practice, however, the speeds of the two beam spotsbecome different from each other due to causes such as a difference inthe characteristics of the lens actuators 12 and 15 and a slight errorbetween the speed signal generators 24 and 34. Due to the differentspeeds, the positions of the two beams are gradually shifted from eachother to generate a difference between the track error signals e and n.

The inter-beam track difference arithmetic device 50 shown in FIG. 1calculates the difference between the track error signals e and n togenerate the inter-beam positional shift signal q. FIG. 6C shows anexample of a waveform of the inter-beam positional shift signal q(converted into an analog value). The inter-beam positional shift signalis level-corrected and input to the power amplifier 37 to drive theobjective lens 14 so as to reduce a positional shift of the beam spot 13with respect to the beam spot 10. That is, if the beam spot 13 has adelay with respect to the beam spot 10 to increase the positional errorbetween the beams, the objective lens 14 is further accelerated tosuppress the positional shift between the beam spots.

In this manner, even if a slight positional shift is generated betweenthe beam spots during the movement of the beam spots 10 and 13, theobjective lens 14 is accelerated or decelerated so as to correct thepositional shift, and the positional shift between the beam spots doesnot increase but is reduced to a small value. Since the relativepositional shift between the beam spots is not integrated even duringmoving the beams for a long distance, the relative positional shiftbecomes almost "0" at a portion near the target track where the beamspeeds are low. Therefore, not only the positional shift between thebeam spots can be reduced during the movement of the beams, but also thetwo beams can reach almost the same track without generating almost nopositional shift between the beam spots at the end of the movement.

As has been described above, according to the present invention, whentwo beam spots are moved between tracks, a relative difference betweenthe positional error signals of the beam spots with respect to a targettrack is calculated to use it as a signal indicating the positionalshift between the two beam spots in order to correct the movement of onebeam. With this operation, the relative positional shift between the twobeam spots which are separately emitted through two objective lensesmounted on the same carriage can be certainly reduced during a trackaccess, and the beams are controlled to almost accurately reach the sametarget track. Therefore, a new, excellent optical disk apparatus capableof moving the two beams at a high speed with a high precision can beprovided.

What is claimed is:
 1. An optical disk apparatus comprising:first andsecond objective lenses respectively for outputting first and secondlight beams on the same track on an optical disk, the first and secondbeams separately recording, reproducing, and erasing information on thesame track; first and second driving means respectively for slightlydriving said first and second objective lenses in a track crossingdirection in accordance with a first and a second positional shiftcorrection signals; a carriage on which said first and second objectivelenses are mounted; a moving mechanism for moving said carriage betweentracks; a fixed optical system for emitting the first and second beamson said optical disk and detecting the beams reflected by said opticaldisk through said first and second objective lenses, and extractingfirst and second track position signals from the detected reflectedbeams of the first and second beams; main control means for controllingan operation of said moving mechanism, and controlling an informationrecording/reproducing/erasing operation using said first and secondobjective lenses through said fixed optical system; track access controlmeans for controlling the movement of said carriage between the tracksbased on the first and second track position signals from said fixedoptical system; first positional shift correction means for counting thefirst track position signal from said fixed optical system to output afirst positional error signal with respect to a target track, andoutputting the first positional shift correction signal to said firstdriving means based on the first positional error signal and the firsttrack position signal; second positional shift correction means forcounting the second track position signal from said fixed optical systemto output a second positional error signal with respect to said targettrack, and outputting the second positional shift correction signal tosaid second driving means based on the second positional error signaland the second track position signal; arithmetic means for calculating adifference between the first and second positional error signals fromsaid first and second positional shift correction means to output aninter-beam positional shift signal; and correction means for correctingone of the first and second positional shift correction signals fromsaid first and second positional shift correction means based on theinter-beam positional shift signal from said arithmetic means.
 2. Anapparatus according to claim 1, wherein said correction means comprisesaddition means for adding the inter-beam positional shift signal fromsaid arithmetic means to one of the first and second positional shiftcorrection signals from said first and second positional shiftcorrection means.
 3. An apparatus according to claim 1, wherein saidfirst positional shift correction means comprises:first positional errorsignal generation means for counting the first track position signalfrom said fixed optical system to generate the first positional errorsignal with respect to the target track; first reference speed signalgeneration means for generating a first reference speed signal fordefining an inter-track moving speed of the first light beam based onthe first positional error signal from said first positional errorgeneration means; first speed signal generation means for generating afirst speed signal indicating a moving speed of the first light beam inthe track crossing direction; and first calculation means forcalculating an error between the first reference speed signal from saidfirst reference speed signal generation means and the first speed signalfrom said first speed signal generation means.
 4. An apparatus accordingto claim 3, wherein said second positional shift correction meanscomprises:second positional error signal generation means for countingthe second track position signal from said fixed optical system togenerate the second positional error signal with respect to the targettrack; second reference speed signal generation means for generating asecond reference speed signal for defining an inter-track moving speedof the second light beam based on the second positional error signalfrom said second positional error generation means; second speed signalgeneration means for generating a second speed signal indicating amoving speed of the second light beam in the track crossing direction;and second calculation means for calculating an error between the secondreference speed signal from said second reference speed signalgeneration means and the second speed signal from said second speedsignal generation means.
 5. An apparatus according to claim 3, whereinsaid second positional shift correction means comprises:secondpositional error signal generation means for counting the second trackposition signal from said fixed optical system to generate the secondpositional error signal with respect to the target track; second speedsignal generation means for generating a second speed signal indicatinga moving speed of the second light beam in the track crossing direction;and second calculation means for calculating an error between the firstreference speed signal from said first reference speed signal generationmeans and the second speed signal from said second speed signalgeneration means.
 6. An apparatus according to claim 1, wherein saidmoving mechanism is constituted by a carriage actuator, and said firstand second driving means are constituted by lens actuators,respectively.